Fix check next key event check in kasp test

Prevent intermittent false positives on slow platforms by subtracting the number of seconds which passed between key creation and invoking 'rndc dnssec -checkds'.

This particularly fails for the step3.csk-roll2.autosign zone because the closest next key event is when the zone signatures become omnipresent. Running 'rndc dnssec -checkds' some time later means that the next key event is in fact closer than the calculated time and thus we need to adjust the expected time by the time already passed.

(cherry picked from commit 262b52a1)